I’ve used my 56mm for Astro before, the attached image is a single exposure straight out of camera, taken with my 56mm & XT-2. I tend to shoot at f2.8 as the vignetting at f1.2 makes Astro panos impossible. Also the chromatic abbreviation at f1.2 is less than ideal. Focus wise I go by the hyperbolics & check with live view (back button to zoom right in on a bright star). Never had any issues with focus drifting but I have had condensation buildup on the from element making it look like the lens is of of focus.
PS, which 56mm are you using as Fujifilm make two, an expensive one and an even more expensive one, mine is the former.